The heating resistor type flow measuring device comprises a sub-intake air passage part which has a flow measuring part including a heating resistor, etc., and an electronically controlled part which controls the heating of the flow measuring part.
The conventional flow measuring device was influenced easily by heat in the engine room because an electronic circuit part is provided on the wall of the intake air pipe in one.
Then, the device (hereafter, slot-in type) that improves the cooling effect of the electronic circuit part by the intake air has been known, in which the electronic circuit part is located in the intake air pipe of the engine to reduce the influence due to such heat.
There is the technology disclosed to the Japanese Patent Application Laid-Open No. 11-14423 as an example of the slot-in type.
In this prior art, the housing part and the sub-intake air passage part are molded separately, and they are firmly fixed side-by-side. At the same time, they are arranged in a longitudinal direction on a plate-like metal base with mechanical strength, and the housing part and the sub-intake air passage part are firmly fixed to the metal base. The fixing among the housing part, the sub-intake air passage part, and the metal base shares two mutually different members.
In the above-mentioned slot-in type, even if the outside of the intake air pipe in the engine room becomes 80° C. for instance, the temperature of air in the intake air pipe is about 20° C. and it is low. Therefore, enough cooling effect is obtained.
Moreover, an individual size in the longitudinal direction is shortened by separately molding the housing part and the sub-intake air passage part, which consist of the plastic material. As a result, the dimensional change when both parts are initially molded is suppressed. Because the housing part and the sub-intake air passage part are fixed on the metal base with the predetermined mechanical strength at the same time as both are fixed, the dimensional changes when assembled and elapsed are suppressed small respectively, and the size accuracy is secured. As a result, the influence on the accuracy of measurement due to the dimensional change is reduced. Therefore, the air flow rate can be measured with a high degree of accuracy.
However, there is still room of the improvement about reliability to the vibration because in the flow measuring device of the above-mentioned slot-in type, the influence due to the resonance with the vibration of the engine is thought.
Moreover, there is still room of the improvement about the method of improving the heat influence on the heating resistor and the temperature sensitive resistor and the intake temperature sensor and the measurement accuracy of the temperature of the intake temperature sensor in order to increase the measurement accuracy of the flow rate.
With regard to these points, the explanation is given hereinafter.
First of all, for the vibration, because the electronic circuit part is arranged in the fluid passage, and a metal base is arranged in the fluid passage for the heat radiation of a transistor for current amplification, the part where mass is large takes the structure thrust out in the intake air pipe from the lower side of the support part in the above-mentioned prior art. As a result, the resonance frequency is decreased easily.
Then, it is possible to increase strength by thickening the thickness of the resin in the support part, and thickening the thickness of the resin of the root part of the housing.
However, the flatness may be decreased due to the generation of the surface sink and the camber, etc. when molded because of the temperature difference generated between the inside and the outside of the resin when cooling. Especially, it is important to prevent surface sink and camber being generated in order to make the lower side of the support part contact closely with the sealant to maintain airtight in the fluid passage.
Further, for the thermometry accuracy, in the above-mentioned prior art configuration, the metal base is arranged in the fluid passage and it is exposed directly to the fluid. As a result, the heat of the internal combustion engine is transferred to the body or the fluid passage structural member, which forms the fluid passage. It is thought that it is easy for the heat to transfer to the heating resistor and the temperature sensitive resistor through the metal plate in the support part to fix it to the body, and further through the metal base and the support terminal.
Especially, at low flow rate, the temperature sensitive resistor, which detects the intake temperature, may detect higher temperature than actual intake temperature. Therefore, more heating currents are supplied to the heating resistor, which keeps temperature difference with the temperature sensitive resistor constant, and a larger flow rate is measured from the actual flow rate. As a result, there is a possibility that the accuracy of measurement is not obtained enough.
Similarly, also for the intake temperature sensor, which measures the intake temperature in the fluid passage, a larger flow rate is measured from the actual flow rate. As a result, there is a possibility that the accuracy of measurement is deteriorated.